Picosecond polarized pump-probe spectroscopy of amylose-iodine

Single-wavelength picosecond polarized pump-probe spectroscopy has been used to study the excited state dynamics of aqueous suspensions of the amylose-iodine complex at wavelengths within the broad absorption band centered at about 635 nm. A single-exponential fit of the magic angle signal at 600 nm gave an average ground state recovery time of 870±50 ps. For excitation at 570 nm a bi-exponential decay with a short component of about 160 ps and a long component of 870 ps gave the best fit of the magic angle signal. The non-exponential decay is attributed to overlapping absorption and relaxation processes due to different polyiodide chains. Since no fluorescence is observed from the complex, the pump-probe signals must be due to relaxation from a long-lived excited state which is not part of the singlet manifold of states. No excitation intensity dependence was observed in the decay kinetics. The anisotropy was constant over the time range of our experiment. The value of the anisotropy was about 0.35±0.04, which is less than the maximum theoretical value of the initial anisotropy, r₁₎,= 0.4. These results are shown to be consistent with a five-level model.